The present invention relates generally to permanent magnet motors that include interior permanent magnets in a rotor.
Permanent magnet brushless (PMBLDC or PMSM) motors may exhibit relatively high torque densities and are therefore useful in industrial drives for high performance applications. Permanent magnet (PM) motors with buried magnets are used in variable speed drives.
The placement of magnets inside the magnet pockets of interior permanent magnet (IPM) motors with rectangular bar magnets is an issue due to the manufacturing tolerances of both magnet bars and magnet pockets. This magnet placement creates ripple torque depending on the slot/pole combination of the motor. For high performance applications, torque ripple is an important challenge for PM motors as it creates vibration and speed pulsation. Moreover, cogging torque minimization in IPM motors is more challenging compared to surface permanent magnet (SPM) motors. IPM motors allow for smaller air gaps and linear skewing. Shaping of the magnet presents design difficulties due to the rectangular shape of the permanent magnets.
Various techniques have been attempted to minimize the cogging torque. Conventional techniques tend to add to the complexity and can negatively impact output torque. In addition, in motors employing sintered magnets, the increased complexity can contribute significantly to cost.
Magnet pole shaping, skewing of rotor magnets or stator structures, step-skewing of rotor magnets, combining slots and poles, magnet shaping, and incorporation of notches in the stator teeth have been employed to minimize cogging torque in PM motors. Unfortunately, however, these conventional techniques cause additional design challenges. For example, the use of segmented stators, while bringing about improvements in slot fill and manufacturing time of the motor, have also given rise to certain undesirable harmonics, such as a large ninth order harmonic attributed to the gaps disposed between stator segments.
Accordingly, it is desirable to have an improved rotor design and techniques for imbedding magnets in rotors of IBPM.